A payday advance is the simplest credit product there is. One disbursement, one repayment, no schedule to amortize, no balance to revolve. It is also the one most likely to fail its own compliance checks, and for a reason that has nothing to do with its simplicity.
The interest is not where the money is
On a 30-day advance, interest is a rounding error next to the fee. The borrower takes MXN 3,000 and repays it once, and what they actually pay for the credit is an origination charge and the tax on it. The annualized cost is enormous, not because the lender is doing anything exotic, but because a fixed charge spread over thirty days annualizes into a very large number.
Advance 3,000.00 Origination 450.00 IVA 72.00 Interest 49.32 Single payment 3,571.32 · day 30
Which means pricing a payday product is not an exercise in choosing a rate. It is an exercise in choosing a fee and then discovering what that fee does to the official cost at every ticket you allow.
The cap bites at the smallest ticket
Because the charge is fixed and the loan is short, the cost of the credit falls as the ticket grows. MXN 6,000 for thirty days carries the same fee structure as MXN 500 and half the annualized cost. The worst corner of a payday product is therefore always the same corner: the smallest amount you are willing to lend, for the shortest term you allow.
That corner is the product. A definition that clears the cap at MXN 6,000 and breaches it at MXN 500 is not a compliant product with an edge case. It is a non-compliant product that happens to look fine in the example someone chose for the meeting. Checks run the minimum, not the illustration.
Days are not a rounded-down month
Short-dated credit is where day-count conventions stop being bookkeeping trivia. Thirty days from the 31st of January is not a month later. A weekend before the due date is not the same as one after it. Accrual on a payday product is computed against exact calendar dates, in integer minor units, because at this term length a single day is a meaningful fraction of the loan.
And when the borrower cannot pay on day 30, whatever happens next is a decision the definition has to have made in advance. A rollover is a new advance with a new fee and a new cost, not the same loan with a later date on it. Products that blur the two are how a short-dated advance quietly becomes a long-dated one nobody priced.